1. Field of the Invention
The present invention relates to a sheet feeding device for feeding a sheet and an image forming apparatus including the sheet feeding device.
2. Description of the Related Art
Conventionally, an image forming apparatus, such as a copying machine or a printer, includes a sheet feeding device, which feeds a sheet stacked on a feeding tray to an image forming unit. Such a sheet feeding device includes a feeding unit, which feeds the sheet stacked on the feeding tray, and a separation unit, which separates one sheet from other sheets.
A separation pad is sometimes used as the separation unit so that one sheet is separated from other sheets by frictional force.
According to the separation method using the above-described separation pad, where frictional force between a feeding roller and the sheet is Frc1, frictional force between the sheet and the separation pad is Frc2, and frictional force between the sheets is Frc3, the sheet can be separated one sheet from other sheets by satisfying a relation of Frc1 >Frc2 >Frc3.
Japanese Patent Application Laid-Open No. 2001-19196 discusses a sheet feeding device, which provides a vibration damping member between a holding member, which holds a separation pad, and a pressure member, which presses the separation pad toward the feeding roller. Further, Japanese Patent No. 2533566 discusses a feeding device, which provides a vibration damping member between a separation pad and a holding member. When such sheet feeding devices are used, transmission of vibration that occurs when the separation pad contacts the sheet to the pressure member can be reduced. Thus, the occurrence of a noise caused by the vibration of the pressure member can be reduced.
The above-described Japanese Patent Application Laid-Open No. 2001-19196 and Japanese Patent No. 2533566, however, do not discuss the stability of the separation performance. The issues of the sheet feeding devices discussed in Japanese Patent Application Laid-Open No. 2001-19196 and Japanese Patent No. 2533566 will be described with reference to FIGS. 9A and 9B. FIG. 9A is a schematic diagram illustrating the configuration of the sheet feeding device according to Japanese Patent Application Laid-Open No. 2001-19196. FIG. 9B is a schematic diagram illustrating the configuration of the sheet feeding device according to Japanese Patent No. 2533566.
As illustrated in FIG. 9A, according to Japanese Patent Application Laid-Open No. 2001-19196, a vibration damping member 27 is arranged between a separation pad holding unit 26 and a separation pad pressure unit 25. The separation pad holding unit 26 holds a separation pad 24. The separation pad pressure unit 25 presses the separation pad 24 toward a feeding roller (not illustrated). As illustrated in FIG. 9B, according to Japanese Patent No. 2533566, the vibration damping member 27 is arranged between the separation pad 24 and the separation pad holding unit 26.
According to the above-described configurations, if thickness of the vibration damping member 27 in a direction of separation pressure is changed, action length of the separation pad pressure unit 25 will be changed, and pressure force of the separation pad pressure unit 25 will be changed. Thus, variations in the thickness of the vibration damping member 27 affect the separation pressure. Further, since the vibration damping member 27 is compressed and deformed by the pressure force applied by the separation pad pressure unit 25, the vibration damping member 27 itself has an elastic force and a viscous force in the direction of the separation pressure. Thus, variations in elastic coefficient and viscosity affect the separation pressure. If the separation pressure is changed, the separation performance of the sheet may be deteriorated.
As described above, according to the configurations discussed in Japanese Patent Application Laid-Open No. 2001-19196 and Japanese Patent No. 2533566, the separation performance may become instable since the variations in the thickness, the elastic coefficient, and the viscosity of the vibration damping member 27 may affect the separation pressure.